1. Field of the Invention
The present invention relates to a high-density circuit module and a method of producing the same, and more particularly, to a high-density circuit module which unites preselected circuits of an electronic device, and also to a method of producing such a high-density circuit module.
2. Description of the Related Art
The inventors of the present application have already proposed a circuit module having thin metal sheet parts such as external lead, shield plate, side frame and shield case. This circuit module is produced by a process which includes the steps of forming the metallic blanks in a developed state, connecting these metallic blanks together in a positional relationship necessary for the assembly, so as to form a single planar developed metallic assembly, placing a printed board on the developed metallic assembly, mounting circuit components on the circuit board and achieving necessary electrical connections, and effecting severance, bending and processing of the connecting portions, This known circuit module is disclosed in, for example, Japanese Patent Unexamined Publication No. 2-73693 which corresponds to U.S. Pat. No. 5,162,971. Referring to FIG. 5 which shows this known circuit module, a shield case 1 and a shield plate 2, which are respectively made of thin sheets of metals such as iron or copper, are jointed together by caulking, fitting or soldering. An external lead 4 has one end received in a hole 9 formed in a circuit board 3 and is connected to a conductor 10 on the circuit board 3 by soldering as at 6. Circuit components 5, 5 mounted on the circuit board 3 have terminals 7 which are connected to conductors 10 on the circuit board 3 by soldering as at 6.
FIGS. 6A and 6B show the process of producing the circuit module shown in FIG. 5. Referring to FIG. 6A, an elongated thin metal sheet 12 has vacant portions partially formed by press work, etching or the like. The thin metal sheet has a main portion 13 which is to form the shield plate 2 in the product, and side frames 14, 15 connected to both sides of the main portion 13, thereby forming slits 16 between there and the portions of the side frames 14, 15. Numeral 18 designates lead portions which are to form the external leads 4 of the product. The lead portions 18 are protruded from a common stem portion 19 so as to connect the lead portions 18 located at both end sides thereof to the above-mentioned main portion 13. Free ends 20 of the lead portions 18 are bent substantially at a right angle. Then, the circuit board 3 is placed on the main portion 13 of the metal sheet 12 as shown in FIG. 7A. In this state, the free ends 20 of the lead portions 18 are inserted into holes 9 formed in the circuit board 3. Then, the connection between the circuit components 5 and the conductors 10 on the circuit board 3, as well as the connection between the lead portions 18 and the conductors 10 on the circuit board 3, is achieved by soldering. The soldering may be conducted by applying a solder paste to all the portions which are to be soldered, and heating and melting the solder paste by, for example, re-flow heating, so that all portions to be connected are soldered at a time. After completion of the soldering, the side wall portions 14, 15 are bent out of the plane of the main part 13. The bending is facilitated by the provision of the slits 16. Subsequently, as shown in FIGS. 8A and 8B, the lead portions 18 are severed from the common portion 19, and the whole structure is covered by the shield case 1, whereby the circuit module as shown in FIG. 5 is obtained.
Achievement of higher packaging density in this type of circuit module poses various requirements to be met. Firstly, it is necessary to reduce the sizes of the circuit components so as to correspondingly reduce the areas of installation of these components. Secondly, the wiring pattern on the circuit board has to be made finer so as to secure the wiring capacity. It is also necessary to mount the circuit components with reduced spacing distances from one another so as to reduce a dead space therein. For these reasons, the prices of the circuit components and the circuit board are raised. Furthermore, the yield of the soldering is reduced due to standing of the chip components during re-flowing of the solder or due to facilitating of generation of solder bridge. Furthermore, since further precise mounting is needed, there is a problem that the number of steps of the mounting process is increased due to the use of a specific mounting device capable of precisely locating and mounting the circuit components. It is also to be pointed out that deterioration in the performance and lack of stability of operation are likely to be caused due to interference between adjacent circuits because of the reduced spacing between these circuit parts. Such deterioration in the performance and unstable operation would be overcome when the circuit module incorporates anti-interference elements such as filters, an internal shield and so forth. Such a solution, however, poses another problem: namely, increase in the material costs and production costs due to increase in the number of parts and the number of steps in the assembly process.